1. Field of the Invention
This invention relates to the measurement of fluid characteristics using two fluidically coupled vibrating cantilevers blades.
2. Prior Art
There are many methods for measuring gaseous pressure including elctrical (ionization gauge), thermal (thermocouple gauge) and mechanical methods. Many of the mechanical methods involve the deflection of a flexible member under the action of a changing pressure coupled with a technique for sensing that deflection. These include Bourdon tubes and a variety of other devices using flexible diaphragms and bellows.
Included in the category of mechanical pressure sensors are those using vibrating elements. Typically, such an element is caused to vibrate at a resonance frequency by piezoelectric or electromagnetic means. As the pressure changes, the restoring force or possibly the damping forces are modified, causing the resonance frequency to change. Using feedback electronics, the vibrator is maintained at a resonance even though the frequency of that resonance is changing. The change in resonance frequency is related to the pressure. Although effective over a restricted range of pressures, these methods generally have a low dynamic range because the resonance frequency is determined by properties of the vibrator as well as the pressure. Thus, as the pressure is reduced below a certain value, the pressure's effect on the frequency falls to zero.
An example of this type of sensor is given by U.S. Pat. No. 4,455,874 to Paros, who describes a force-sensitive resonator mounted in several different ways so that a force is applied to the resonator as pressure is changed, thereby changing the resonance frequency. Additionally, the Paros patent describes a large number of known vibrational structures for pressure sensing. U.S. Pat. No. 4,409,840 to Roberts describes a vibrating cantilever blade which is excited to vibrate. After a temporary excitation, the driving force is stopped and the temporal decay (time constant) of the vibrational amplitude is observed. This time constant is related to the external pressure. However, like those techniques which monitor a frequency shift, the decay is also strongly influenced by structural factors with the result that for pressures in the range where these factors are dominant, sensitivity to pressure is very low. As described in these teachings, many of these vibrational techniques also have a sensitivity to the atomic mass of the gas under consideration. U.S. Pat. No. 3,187,579 to Ferrau et al shows that the vibrational approach also applied to the measurement of liquid pressures.
It would be desirable to have the entire vibrational amplitude of the sensing vibrator device to be a function of gas pressure and to have the device be sensitive over a very wide pressure range. A simple, compact and practical device would also be desirable. These are some of the problems this invention overcomes.